Lithography apparatuses are used for example in the production of integrated circuits or ICs in order to image a mask pattern in a mask onto a substrate, such as e.g. a silicon wafer. In this case, a light beam generated by an illumination device is directed through the mask onto the substrate. An exposure lens consisting of a plurality of optical elements serves for focusing the light beam on the substrate.
The extent of the smallest structure elements which can be realized on the wafer is proportional to the wavelength of the light used for exposure and inversely proportional to the numerical aperture of the optical elements (lens elements or mirrors) used for beam shaping. In order to meet the requirements for ever smaller structures, there was a development toward light sources having ever shorter wavelengths, through to EUV (Extreme UltraViolet) light sources having a wavelength of less than 30 nm, in particular less than 13 nm or even less than 5 nm. Such short wavelengths make it possible to image extremely small structures on the wafer. Since light in this wavelength range is absorbed by atmospheric gases, the beam path of such EUV lithography apparatuses is situated in a high vacuum. Furthermore, there is no material that is sufficiently transparent in the wavelength range mentioned, for which reason mirrors are used as optical elements for shaping and guiding the EUV radiation.
In order to improve the imaging quality of microlithography apparatuses, it has been proposed to deliberately influence the polarization state of the light used for image generation. In this regard, the published patent application DE 101 24 803 A1 proposes a polarizer for generating a light beam having a predefined distribution of polarization states over its cross section, and a microlithography apparatus comprising such a polarizer.
Furthermore, the mirrors used for beam guiding can also influence the polarization since they can have a different reflectivity for p-polarized and s-polarized light, which can lead to image aberrations or other effects to be avoided.
In order to monitor and, if appropriate, optimize the polarization of light in the microlithography apparatus, a polarization measuring technique for measuring the polarization state is necessary. WO 2010/105757 A1, DE 10 2009 021 096 A1, US 2007/182969 A1, US 2011/032502 and US2010/208264 disclose methods and devices for measuring the polarization state in projection exposure apparatuses, wherein all the documents propose the use of transmissive elements such as, for example, beam splitters, polarizers, retardation elements and the like. However, the use of transmissive elements is suitable only to a limited extent for the EUV range, since said elements lead to a reduction of the light intensity, with the result that longer integration times are necessary for a measurement. Furthermore, said documents use in some instances methods in which polarizers and the like are rotated in different orientations (see e.g. US2010/208264), which is likewise time-consuming.
The known polarization measuring devices are therefore relatively complex and, e.g. on account of their use of transmissive elements, suitable only to a limited extent for use in the EUV range.
EP 1306665A2 discloses an optical apparatus for measuring polarization including a rotary polarizer including a set of mirrors repeating three or more reflections and arranged such that the optical axes of incident and of the outgoing light are aligned with the same straight line. Polarization measurement devices with rotating reflective plates are also disclosed in DE 10347978 A1 and U.S. Pat. No. 4,725,145.